Photopolymerizable elastomeric mixture and recording material containing this mixture, for producing ozone-resistant flexographic printing forms

ABSTRACT

A photopolymerizable elastomeric mixture for producing ozone-resistant flexographic printing forms is disclosed which contains an elastomeric binder, an olefinically unsaturated compound polymerizable by free radicals, a photoinitiator, and a polyglycol ether of the general formula I ##STR1## in which R 1  and R 2  are alkyl, especially (C 1  -C 3 )-alkyl, 
     R 3  is hydrogen or alkyl, especially (C 1  -C 3 )-alkyl, 
     A, B, C and D can be identical or different and are oxa-n- or -iso-alkylene, especially oxa-n- or -iso-(C 1  -C 5 )-alkylene and 
     l+m+n+o is 3 to 40. 
     A recording material containing the photopolymerizable mixture as well as a process for producing a flexographic printing form are likewise described. The flexographic printing form obtained shows improved ozone resistance as compared with forms of the state of the art.

This is a division of application Ser. No. 08/323,167, filed Oct. 13,1994, now allowed, which was a continuation of Ser. No. 08/407,614 filedApr. 16, 1993, since abandoned, which was a continuation of Ser. No.07/450,077 filed Dec. 13, 1989, since abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a photopolymerizable elastomericmixture for producing ozone-resistant flexographic printing forms,containing an elastomeric binder, an olefinically unsaturated compoundpolymerizable by free radicals, and a photoinitiator.

Elastomers, in particular those based on butadiene or isoprene, havebeen known for a long time for production of flexographic printing formsand are described, for example, in DE-B 2,215,090 (=U.S. Pat. No.4,423,135), DE-A 2,138,582 (=GB-A 1,358,062), 2,223,808 (=GB-A1,395,822), DE-B 2,815,678 (=GB-A 1,577,706), DE-A 2,456,439 (=U.S. Pat.No. 4,162,919) and 2,942,183 (=U.S. Pat. No. 4,320,188). It was found,however, that flexographic printing forms containing these elastomersbecome brittle with even the slightest presence of ozone and showcracks.

Ozone, which represents a risk to these plates, is formed both duringimagewise exposure of the photopolymerizable elastomeric layer,especially if UV light is used, and during use of the exposed anddeveloped printing forms. In the latter case, particularly high ozoneconcentrations arise, especially when flexible packaging materials suchas paper or plastics are to be printed. This is because, in order toensure sufficiently strong adhesion of the printing ink, for example, tothe plastic films, the latter are corona-treated before printing. Sinceconsiderable quantities of ozone are formed during the corona treatmentand the treatment is most efficient when it takes place immediatelybefore the printing step, the printing form is contacted with aconsiderable amount of ozone resulting from this process. Thesusceptibility of the developed printing form to ozone is furtherincreased by the fact that a post-treatment of the printing forms withhalogen, in particular with bromine, is carried out in most cases, inorder to reduce their tackiness.

To improve the resistance of the printing form to ozone which formscracks and promotes brittleness, various approaches have been followed.

According to DE-A 2,215,090, ozone resistance can be improved byadditions to the photopolymerizable mixture. Microcrystalline wax andparaffin wax, dibutylthiourea, 1,1,3,3-tetramethylthiourea, norbornene,N-phenyl-2-naphthylamine, unsaturated vegetable oils, ethylene/vinylacetate copolymers, polyurethanes, chlorinated and/or chlorosulfonatedpolyethylenes, chlorinated ethylene/methacrylic acid copolymers,polypentadienes, furfural-derived resins, ethylene/propylidene rubbers,diethylene glycol esters of resins and copolymers of α-methylstyrenewith vinyltoluene are proposed as additives.

The disadvantage of this method is that the additives are frequentlyinadequate to ensure the required resistance to ozone or, if theeffective concentration of these additives is increased, thephotopolymerizable mixture shows such cloudiness that satisfactoryimaging is no longer feasible.

Another method for protecting a flexographic printing form from ozone isdescribed in DE-A 3,512,632 (=U.S. Pat. No. 4,680,251). This method isnot based on any additions to the photopolymerizable mixture, butdescribes a post-treatment of the printing form, which has beendeveloped and treated with halogen, with certain polyglycol compounds.The polyglycol compounds described are either not etherified or onlymono-etherified.

A major disadvantage of this method is the required additional treatmentstep. Also, the treatment reagents described leave much to be desiredwith respect to the achievable ozone resistance.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aphotopolymerizable mixture especially suited for production offlexographic printing forms, having improved ozone resistance and, atthe same time, requiring no additional treatment step. The ozoneresistance achieved should not have an adverse effect on the imagequality.

These and other objects according to the invention are achieved by aphotopolymerizable elastomeric mixture for producing an ozone-resistantflexographic printing form, comprising an elastomeric binder; anolefinically unsaturated compound polymerizable by free radicals; and aphotoinitiator;

wherein the mixture comprises at least one polyglycol ether of thegeneral formula I ##STR2## in which R₁ and R₂ are alkyl, especially (C₁-C₃)-alkyl,

R₃ is hydrogen or alkyl, especially (C₁ -C₃)-alkyl,

A, B, C and D can be identical or different and are oxa-n- or-iso-alkylene, especially oxa-n- or -iso-(C₁ -C₅)-alkylene, and

l+m+n+o is 3 to 40.

The objects of the invention are further achieved by a recordingmaterial producing an ozone-resistant flexographic printing form,consisting essentially of a carrier; and a layer of thephotopolymerizable elastomeric mixture.

In addition, the present invention provides a process for producing anozone-resistant flexographic printing form, comprising the steps of:

a) preparing a photopolymerizable mixture according to the invention bya process selected from the group consisting of

extruding the constituents in a single-screw or twin-screw extruder withsubsequent final forming in a polishing stack or calender,

using a roller head process, and

dissolving the constituents in an organic solvent, followed bycrosslinking at elevated temperatures and then pressing,

b) applying or laminating the layer or solution prepared in a) to acarrier which has optionally been treated to provide an adhesion layer,

c) imagewise exposing with actinic light,

d) removing the unexposed and uncrosslinked layer areas with a developersolvent, and

e) drying the flexographic printing form.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The photopolymerizable elastomeric mixture according to the presentinvention comprises at least one polyglycol ether of the followinggeneral formula I ##STR3## in which R₁ and R₂ is alkyl, especially (C₁-C₃)-alkyl,

R₃ is hydrogen or alkyl, especially (C₁ -C₃)-alkyl,

A, B, C and D can be identical or different and are oxa-n- or-iso-alkylene, especially oxa-n- or -iso-(C₁ -C₅)-alkylene and

l+m+n+o is 3 to 40.

It is preferred when m and o are greater than 0 and o is less than orequal to m, and R₃ is (C₁ -C₃)-alkyl. It is particularly preferred thatboth o and m be equal to 1.

If o is not 0, D is preferably identical to A or C, and B is in thiscase an oxa-n-alkylene radical, especially an oxa-n-(C₃ or C₅)-alkyleneradical. It is particularly preferred, however, when o is 0 and the sumof 1, m and n is between 3 and 40.

In a further preferred embodiment, A is identical to C but differentfrom B. In this case, it is preferred that A and C is an oxa-n-(C₂-C₄)-alkylene radical and B is an oxa-n- or -iso-(C₂ or C₃)-alkyleneradical. An oxa-isoalkylene radical is especially preferred for B.

It is also preferred when m is 0 in addition to o being 0, and the sumof 1 and n reaches 3-40. In this case, A and C are oxa-n- or -iso-(C₁-C₅)-alkylene, in particular oxa-n- or -iso-(C₂ or C₃)-alkylene, A or Cdiffering at least in such a way that they are not conjointly anoxa-n-alkylene radical or an oxa-iso-alkylene radical.

Rather, however, that variant is preferred in which l and m, or m and n,are each equal to 0 (o is likewise 0 in this case). In this case, C andA is oxa-n- or -iso-(C₂ -C₅)-alkylene, especially oxa-n- or -iso-(C₂ orC₃)-alkylene.

R₁ and R₂ are in all cases (C₁ -C₃)-alkyl. In particular, R₁ and R₂ areidentical and particularly preferably are methyl.

The mean molecular weight of the polyglycol ether according to thegeneral formula I is about 100 to 1,400, especially about 200 to 1,200and particularly preferably about 200 to 600.

The photopolymerizable elastomeric mixture according to the inventioncontains about 0.1 to 40% by weight, preferably about 0.5 to 20% byweight, of at least one additive according to the general formula I, ineach case relative to the elastomeric binder contained in the mixture.The addition of only one compound according to the general formula I isparticularly preferred.

Polymers of conjugated aliphatic dienes having 4 to 5 carbon atoms aresuitable as the elastomeric binders. Particularly suitable are naturalrubbers, homopolymers or copolymers of butadiene and isoprene,copolymers of butadiene and/or isoprene with other monomers such asstyrene, vinyltoluene, acrylonitrile or (meth)acrylic acid esters, forexample nitrile rubbers, random copolymers of styrene/butadiene,styrene/isoprene and styrene/isoprene/butadiene, or also blockcopolymers of styrene monomers and butadiene and/or isoprene, having astyrene content of about 10 to 50% by weight. Elastomeric binders ofthis type are described in DE-A-2,215,040.

The photopolymerizable mixture according to the invention contains ingeneral about 20 to 98% by weight, preferably about 30 to 95% by weight,of at least one of the said elastomeric binder types. It also containsat least one olefinically unsaturated compound polymerizable by freeradicals, and at least one photoinitiator.

Suitable monomers having one or more polymerizable olefinic double bondsare especially esters and amides of acrylic and methacrylic acid.Examples are the compatible monoacrylates and diacrylates andmonomethacrylates and dimethacrylates of monohydric or polyhydricalcohols such as ethylene glycol, di-, tri-, tetra- or polyethyleneglycols, the latter preferably with 10 to 15 ethylene glycol units,1,3-propanediol, 1,6-hexanediol, dodecanediol, glycerol,1,1,1-trimethylolpropane, 1,2,4-butanetriol or pentaerythritol, forexample ethylene glycol monomethacrylate, 1,3-propanediolmonomethacrylate, glycerol monoacrylate and diacrylate,1,2,4-butanetriol monomethacrylate, hexanediol diacrylate, hexanedioldimethacrylate, dodecanediol diacrylate, 2-ethylhexyl acrylate, laurylmethacrylate, stearyl methacrylate, pentaerythritol triacrylate,polyethylene glycol methyl etheracrylate, tetradecaethylene glycoldimethacrylate or the triether of glycerol and 3 mol ofN-methylolacrylamide or N-methylolmethacrylamide. Alkenylphosphonic andalkenylphosphinic acid esters according to German Patent Application P3,817,424.3 can also be used. The quantity of monomers is in generalabout 1 to 70% by weight, preferably about 2 to 50% by weight, relativeto the non-volatile constituents of the mixture.

The photoinitiators used for the mixture according to the invention canbe known compounds which have an adequate thermal stability during theprocessing of the recording materials and show adequate formation offree radicals during exposure with initiation of the polymerization ofthe monomers. They should absorb light in the wavelength region fromabout 250 to about 500 nm with the formation of free radicals. Examplesof suitable photoinitiators are acyloins and derivatives thereof, suchas benzoin, benzoin alkyl ethers, for example benzoin isopropyl ether,vicinal diketones and derivatives thereof, for example, benzil, benzilacetals such as benzil dimethyl ketal, fluorenones, thioxanthones,polynuclear quinones, acridines and quinoxalines; and alsotrichloro-methyl-s-triazines, 2-halogenomethyl-4-vinyl-1,3,4-oxadiazolederivatives, halogeno-oxazoles substituted by trichloromethyl groups,carbonylmethylene heterocyclic compounds containing trihalogenomethylgroups according to DE-A 3,333,450, acylphosphine oxide compounds suchas are described, for example, in DE-A 3,133,419, and otherphosphorus-containing photoinitiators, for example the6-acyl-(6H)-dibenzo[c,e][1,2]oxaphosphorine-6-oxides described in GermanPatent Application P 3,827,735.2, especially6-(2,4,6-trimethylbenzoyl)-(6H)-dibenzo[c,e]-[1,2]oxaphosphorine-6-oxide.The photoinitiators can also be used in combination with one another orwith coinitiators or activators, for example with Michler's ketone andderivatives thereof or 2-alkyl-anthraquinones. The quantity ofphotoinitiator is in general about 0.01 to 10% by weight, preferablyabout 0.5 to 5% by weight, relative to the photopolymerizable mixture.

It is frequently of advantage also to add further auxiliaries andadditives to the photopolymerizable mixture, for example, inhibitors ofthermal polymerization such as hydroquinone and its derivatives,2,6-di-tert.-butyl-p-cresol, nitrophenols, nitrosamines such asN-nitrosodiphenylamine or salts of N-nitrosocyclohexyl-hydroxylamine,for example, alkali metal salts or aluminum salts thereof. Furtherconventional additives are dyes, pigments, plasticizers, antihalationagents, antioxidants, crosslinking agents, regulators, fillers, flowagents and further auxiliaries which improve the function of the layer.

The mixture according to the invention can be used for producing reliefand flexographic printing plates by casting from a solution with anorganic solvent and by kneading at elevated temperatures and subsequentpressing. Production is also possible by extruding in a single-screw ortwin-screw extruder with subsequent final forming by means of apolishing stack or a calender to give layers of a thickness of about 0.1to 10 mm, preferably from about 0.4 to 6 mm. Production by the rollerhead process is likewise possible. The layer produced in the waydescribed can be laminated to the surface of a suitable carrier, or asolution of the mixture according to the invention can be applied to alayer carrier.

In addition to the production of relief printing plates, the mixtureaccording to the invention can also be used, for example, for theproduction of flexographic printing plates, gravure printing cylinders,screen printing stencils and photoresists.

Depending on the intended use, examples of suitable carriers arepolyester films, steel sheets or aluminum sheets, copper cylinders,screen printing stencil carriers, plies of foam material, rubber-elasticcarriers or printed circuit boards. It can also be advantageous to applyto the light-sensitive recording layer a covering layer or protectivelayer, for example a thin layer of polyvinyl alcohol, or a covering filmwhich can be peeled off, for example, of a polyethylene terephthalate orpolyamide. Moreover, precoating of the carrier can be advantageous. Theadditional layer between the carrier and the light-sensitive layer can,for example, be effective as an antihalation layer or as an adhesionlayer.

The recording material according to the invention can be exposedimagewise with actinic light from light sources such as mercury vaporlamps or fluorescent tubes, the emitted wavelength preferably beingbetween about 300 and 420 nm.

Removal of the unexposed and uncrosslinked layer areas can be effectedby spraying, washing or brushing with chlorinated hydrocarbons, esters,aromatic compounds or phenol ethers, such as are described inDE-A-2,215,090 and in German Patent Applications P 3,836,402.6, P3,836,403.4 and P 3,836,404.2, as yet unpublished. Minor amounts ofantifoam or water-miscible organic solvents, for example, loweraliphatic alcohols, can also be added to the solution. Depending on thebinder used, development is also possible with water or aqueoussolutions.

Expediently, the developed relief forms are dried at temperatures up toabout 120° C. and, if appropriate, post-exposed simultaneously orsubsequently with actinic light. After drying of the flexographicprinting form, the latter can be post-treated with a halogen-containingsolution such as, for example, a solution of bromine or chlorine. Apost-treatment with actinic light, especially light of a wavelength ofless than about 300 nm, is likewise possible. Before the imagewiseexposure, the whole area can also be briefly exposed with actinic lightfrom the opposite side.

The recording material according to the invention is particularlysuitable for producing printing forms, especially letterpress printingforms or relief printing forms, which are particularly suitable forflexographic printing.

The invention is explained by the examples which follow. In theexamples, p.b.w. means parts by weight; unless otherwise stated, g:cm³is 1:1.

EXAMPLE 1 (COMPARISON EXAMPLE)

A photopolymerizable elastomeric mixture composed of

90 parts by weight of a styrene/isoprene/styrene three-block copolymerhaving a styrene content of 15% by weight,

8 parts by weight of hexanediol-diacrylate,

2 parts by weight of benzil dimethyl ketal and

0.3 part by weight of 2,6-di-tert.-butyl-4-methylphenol in

100 parts by weight of toluene

was prepared.

The mixture was poured out to give a 6 mm thick layer onto a 125 μmthick, biaxially stretched and heat-set polyester film in a metal frame.After evaporation of the toluene, a 125 μm thick, biaxially stretchedand heat-set polyester film, provided with an adhesion layer, wasapplied to the free surface of the now 3 mm thick solid photopolymerlayer, and the resulting three-layer element was pressed for 5 minutesunder a pressure of 400 N/cm² at 110° C. in a plate press, using 2.7 mmthick spacers.

After the polyester film which had not been treated with an adhesionpromoter had been peeled off, an overcoat of polyamide in a thickness of3-4 μm was laminated on in its place. First, the whole area of thephotopolymer layer was exposed through the remaining polyester film. Forthis purpose, the multi-layer element was exposed for 10 seconds to theradiation of a commercially available UV-A flat exposure apparatus(spectral region 320-400 nm, radiation intensity on the photopolymersurface: 14 mW/cm²). Imagewise exposure was then carried out with thesame radiation source for 30 minutes, but from the opposite side of themultilayer element. Development was carried out with perchloroethylene.After the resulting flexographic printing form had been dried, it waspost-treated for 5 minutes with a 0.4% by weight aqueous solution ofbromine, followed by rinsing with water for 10 minutes.

The flexographic printing form was then post-exposed for 10 minutes in acommercially available flat exposure apparatus.

The flexographic printing form was then clamped on to a cylinder of 7 cmdiameter and subjected to ozone-containing air (0.5 ppm of ozone) in achamber. The plate was assessed, on the one hand, for the time at whichthe first cracks appear (cracks visible with the naked eye) and, on theother hand, for the number of cracks which had formed after ozonetreatment for 20 hours. The results are summarized in Table 1.

EXAMPLE 2 (COMPARISON EXAMPLE)

A photopolymerizable elastomeric mixture corresponding to Example 1 wasprepared, but with an addition of 5 parts by weight of dipropyleneglycol monomethyl ether. The elastomeric mixture was, as indicated inExample 1, processed further to give a flexographic printing plate andsubjected in the same way to the test in the ozone chamber. The resultis comparable with that from Example 1. The additive does not appear toexhibit an effect in the plate. The result can be seen from Table 1.

Example 3

A photopolymerizable elastomeric mixture corresponding to Example 1 wasprepared, but with an addition of 10 parts by weight of tetraethyleneglycol dimethyl ether. The elastomeric mixture was, as indicated inExample 1, processed further to give a flexographic printing plate andsubjected in the same way to the test in the ozone chamber. The resultscan be seen from Table 1.

EXAMPLE 4

A photopolymerizable elastomeric mixture corresponding to Example 1 wasprepared, but with an addition of 5 parts by weight of polyethyleneglycol dimethyl ether having a mean molecular weight of 250. Theelastomeric mixture was, as indicated in Example 1, processed further togive a flexographic printing plate and subjected in the same way to thetest in the ozone chamber. The results are summarized in Table 1.

EXAMPLE 5

In place of polyethylene glycol dimethyl ether of molecular weight 250,5 parts by weight of polyethylene glycol dimethyl ether of a meanmolecular weight of 500 were used in this example. In other respects,the procedure corresponded to that of Example 4. The results are shownin Table 1.

EXAMPLE 6

A photopolymerizable elastomeric mixture composed of

90 parts by weight of a styrene/isoprene/styrene three-block copolymerhaving a styrene content of 15% by weight,

8 parts by weight of hexanediol diacrylate,

2 parts by weight of benzil dimethyl ketal,

0.3 part by weight of 2,6-di-tert.-butyl-4-methylphenol and

5 parts by weight of polyethylene glycol dimethyl ether (mean molecularweight 250)

was prepared.

The mixture was kneaded in a kneader for 15 minutes at 150° C. Themixture was then applied to a 125 μm thick, biaxially stretched andheat-set polyester film provided with an adhesion layer, and theresulting layer was covered by the same polyester film, but withoutadhesion layer. The multilayer element produced was pressed for 2minutes at 110° C. in a plate press, using 2.7 mm thick spacers. Afterthe polyester film not coated with an adhesion promoter had been peeledoff, further processing was carried out corresponding to Example 1. Theresult of this test is likewise demonstrated in Table 1.

Table 1

Treatment of the flexographic printing form with ozone-containing air(0.5 ppm of ozone)

    ______________________________________                                                    Start of crack                                                                              Crack frequency                                     Example     formation     after 20 hours                                      ______________________________________                                        1           2 minutes     many                                                2           2 minutes     many                                                3           1 hour        few                                                 4           1 hour        few                                                 5           3 hours       few                                                 6           2 hours       few                                                 ______________________________________                                    

Table 1 demonstrates that the photopolymerizable elastomeric mixtureaccording to the invention with the polyglycol ether according togeneral formula I produces flexographic printing forms which, in asurprising manner, are substantially more resistant to ozone-containingair than flexographic printing forms which have hitherto been known tothose skilled in the art.

What is claimed is:
 1. A process for producing an ozone-resistantflexographic printing form, comprising the steps of:a) preparing aphotopolymerizable layer comprising an elastomeric binder; anolefinically unsaturated compound polymerizable by free radicals; and aphotoinitiator to form a mixture; wherein the mixture comprises at leastone polyglycol ether of the general formula I: ##STR4## in which R₁ andR₂ are (C₁₋₃)alkyl; R₃ is hydrogen or (C₁₋₃)alkyl A, B, C, and D can beidentical or different and are selected from oxa-n-alkylene oroxa-isoalkylene and wherein l+m+n+o is 3 to 40:by (1) an extrusionprocess comprising extruding the mixture in a single screw or twin-screwextruder with subsequent final forming in a polishing stack or calender;or (2) forming a layer out of the mixture using a roller head process or(3) dissolving the mixture in an organic solvent, followed bycross-linking at elevated temperature and then pressing into a layer; b)applying or laminating the photopolymerizable layer prepared in step a)to a carrier, c) imagewise exposing the photopolymerizable layer withactinic light to produce polymerized and non-polymerized areas, d)removing the unexposed and non-polymerized areas of thephotopolymerizable layer with a developer solvent to form a developedflexographic printing form, and e) drying the developed flexographicprinting form to form an ozone resistant flexographic printing form. 2.The process as claimed in claim 1, wherein, before the imagewiseexposure step c), the photopolymerizable layer is overall exposedthrough the carrier.
 3. The process as claimed in claim 1, wherein,after drying in step (e), the flexographic printing form is post-treatedwith a halogen solution, or post exposed with actinic light.
 4. Aprocess as claimed in claim 1, additionally comprising the step ofcovering the photopolymerizable layer with a protective layer.
 5. Aprocess according to claim 1 wherein the carrier layer has an adhesionlayer which is adjacent to the photopolymerizable layer.